api: fix compatibility of linux/in.h with netinet/in.h

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This fixes breakage to iproute2 build with recent kernel headers
caused by:
   commit a263653e


   Author: Pablo Neira Ayuso <pablo@netfilter.org>
   Date:   Wed Jun 17 10:28:27 2015 -0500

   netfilter: don't pull include/linux/netfilter.h from netns headers

The issue is that definitions in linux/in.h overlap with those
in netinet/in.h. This patch solves this by introducing the same
mechanism as was used to solve the same problem with linux/in6.h

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>